The invention relates to a laser apparatus with a multipass resonator. In the beam path between two resonator end mirrors, such a laser has at least two beam folding mirrors with confronting mirror surfaces, at least one of which is limited in its width and at least one of the beam folding mirrors has a beam discharge area.
A laser with multipass resonator is disclosed, for example, in German Patent No. 35 15 679. This laser has an internal electrode which is coaxially surrounded by a tubular external electrode. The annular space between the two electrodes is defined axially by two beam folding mirrors. Such multipass resonators have the advantage that a great resonator length can be achieved with a short length of the laser, since the beam is repeatedly reversed between the beam folding mirrors. What is problematical in such resonators is the adjustment of the two mirrors to one another and the precise creation of the individual reflective surfaces on the beam folding mirrors which are spherical mirrors, such that the laser beam will be reflected between the two mirrors without any great deviation.
Through the use of spherical beam folding mirrors, the individual points at which the laser beam strikes the beam folding mirrors are situated on a circle with the radius r or generally on an ellipse. The stability is insensitive to r, the magnitude of r is not defined. Since the beam folding mirrors are rotationally symmetrical, the position of the points of impingement is not established by the circumference, i.e., basically any number of modes can vibrate. What is desired, however, is a mode of a low order, preferably a 00 basic mode. Even slight deviations from a precise adjustment will cause the beam to be reflected undefined at the mirror surfaces and thus undesired modes can be excited. To suppress undesired modes, it is common practice to provide mode masks in the form of bored disks as additional components in front of the beam folding mirrors, which permit the beam to strike only on predetermined areas of the beam folding mirror.
In a multipass resonator the laser beam strikes the beam folding mirror at a slight angle. To obtain a circular mode the mark must accordingly have a corresponding contour or must be as near as possible to the reflective surfaces, and cannot be positioned at any desired point in the resonator as it can in the case of a nonfolded resonator.
A disk with holes, as used for the known lasers with multiplass resonators, has the disadvantage that the surfaces of the mirror can be damaged by its installation. Also the intense radiation can vaporize material from the mask onto the mirror surfaces.